1. Field of the Invention
The present invention relates to a vehicular brake system in which a boosting effect of a brake booster for assisting a brake pedal depressing operation to enhance a braking performance is compensated when the brake booster has failed.
2. Related Art
Conventionally, a brake booster for boosting a brake pedal depressing force (for implementing a so-called brake assist operation) is mounted in a vehicle in order to reduce a driver's load to depress the brake pedal.
As a brake booster of this type, there has been known a vacuum booster for boosting the brake pedal depressing force. The vacuum booster applies a boosted large force to a master cylinder by using the difference between a negative pressure of an intake side of an engine for example and an atmospheric pressure. Because the vacuum booster cannot exhibit a desired boosting effect when it fails, there has been discussed lately control for determining whether or not the vacuum booster fails and for executing another brake assist operation for increasing brake fluid pressure by driving a pump and others to enhance a braking force when the vacuum booster is determined to fail.
However, in the above-mentioned technology in which the vacuum booster and another brake assist device are used together, there has been a possibility that the vehicle starts to move by its gravity when the vacuum booster has failed and the vehicle is stopped on a steep slope for example. The reason is that, when the brake assist operation for increasing the brake fluid pressure by driving the pump and the like is finished in response to the vehicle stopping, the braking force due to another brake assist device becomes small rapidly, and because the vacuum booster fails, a driver cannot depress the brake pedal sufficiently to generate braking force for keeping the vehicle resting on the slope.
Accordingly, it is a first object of the present invention to solve the above-mentioned problem by providing a vehicular brake system which can perform favorable brake control at the time of the vehicle stopping by use of another brake assist device when the boosting effect of the brake booster cannot be fully exhibited.
There has been also studied a so-called pressure amplifying assist brake (PAB) for increasing the wheel cylinder pressure by use of a plurality of cut-off valves and a pump even when the vacuum booster has failed. In detail, the wheel cylinder is increased by cutting off a flow path from a master cylinder to the wheel cylinder by a master cut-off valve (SMC valve), by opening a flow path from the master cylinder to an intake side of a pump by a cut-off valve (SRC valve) and by driving the pump in this state.
The magnitude of the pressure increased by the PAB is controlled by the opened/closed state of the SMC valve and the SRC valve or by the driving state of the pump. When the pressure increase is controlled by the opened/closed state of the SMC valve, or more concretely, when the SMC valve is turned OFF (when the flow path is opened) for example, there is a case where pulsations of brake fluid pressure occur.
When it is detected by the fluid pressure sensor that the fluid pressure has been increased (due to the increase of pressure caused by the instantaneous pulsation, not due to the normal increase of pressure) for example, the SMC valve, the SRC valve, the pump and others are driven wastefully to reduce the fluid pressure to an adequate level. That is, there has been a problem that when the pulsations of the fluid pressure occur, inadequate control for decreasing and increasing the fluid pressure is carried out.
Accordingly, it is a second object of the present invention to provide a vehicular brake system which can control the increase/decrease of the brake fluid pressure adequately even when the pulsations of the fluid pressure occur.